Zeolites with high silica to alumina ratios, i.e., of at least six, are desirable because of their particular catalytic selectivity and their thermal stability; the latter is a property particularly important when the zeolite is used as catalyst or in adsorption procedures wherein exposure to high temperatures would be expected. Although zeolites having silica to alumina ratios of less than six can be readily synthesized by a variety of methods, as disclosed, e.g., in U.S. Pat. Nos. 2,882,244 and 4,178,352. Methods for preparing faujasite polymorphs of higher ratios generally involve several weeks of crystallization and result in poor yields of product, as reported by Kacirek, J. Phy. Chem., 79 1589 (1975). One successful method results in a high silica faujasite, CSZ-3, that contains CS.sup.+ cations trapped within the sodalite cage subunits of the structure and has a composition (Na, Cs).sub.2 O: Al.sub.2 O.sub.3 : 5-7 SiO.sub.2 ; See U.S. Pat. No. 4,333,859. A similar material is CSZ-1 (U.S. Pat. No. 4,309,313), having a similar composition, but having a rhombohedral structural distortion (Treacy et al., J. C. S. Chem Comm., p 1211, (1986)).
The use of quaternary ammonium salts as templates or reaction modifiers in the preparation of synthetic crystalline aluminosilicates (zeolites), first discovered by R. M. Barrer in 1961, has led to preparation of zeolites with high silica to alumina ratios which are not found in nature. For example, U.S. Pat. No. 4,086,859 discloses preparation of a crystalline zeolite thought to have the ferrierite structure (ZSM-21) using a hydroxethyl-trimethyl sodium aluminosilicate gel. A review provided by Barrer in Zeolites, Vol. I, p. 136 (October, 1981) shows the zeolite types which are obtained using various ammonium organic bases as cation. In addition, Breck, Zeolite Molecular Sieves, John Wiley (New York, 1974), pp. 348-378, provides a basic review of zeolites obtained using such ammonium cations in the synthesis thereof, as does a review by Lok et al (Zeolites, 3, p 282 (1983)).
The Si/Al ratios of a variety of readily synthesized NaY materials can be increased by a wide range of chemical or physical chemical treatments. However, these processes usually involve removal of Al from the zeolite framework and creation of a metastable defect structure, followed by filling the defects with Si from another part of the structure by further chemical treatments or hydrothermal annealing. Typical treatments use steam, e.g., U.S. Pat. No. 3,293,192; acid leaching, e.g., U.S. Pat. No. 3,506,400; treatments with EDTA, e.g., U.S. Pat. No. 4,093,560; treatment with SiCl.sub.4 (Beyer and Belenykaja, Catalysis by Zeolites S, p. 203 (1980), Elsevier Press.); treated with CHF.sub.3, i.e., U.S. Pat. No. 4,275,046; or treated with other chemicals. The products are often called `ultra stable` faujasites (cf. Maher and McDaniel Proceedings Intl. Conference on Molecular Sieves, London, 1967, Ed. R. M. Barrer) because of their very high thermal and hydrothermal stability. However, such chemical processing often yields variable products, requires multi-step processing, often using highly corrosive environments, and usually involves a yield debit in the form of partly collapsed or blocked zeolite product. Few of the modified materials have the product quality of the starting sample because the process of modification involves partial destruction of the lattice and/or deposition of detrital reaction products within the pores of the structure. The existence of a secondary pore structure within the mesopore range has been reported (Lhose et al, Zeolites, 4 p 163, (1984)). Very recently "framework exchange" treatments of NaY using ammonium silicon hexafluoride (Eur. Pat. Appln. No. 008221) have yielded higher silica faujasite materials. However, these seem to contain undesirable residual F.sup.- anions (presumably replacing (OH).sup.-) and are somewhat limited in the degree of possible modifications using single treatments. Any chemical dealumination treatment may be expected to react more preferentially with the crystal exterior surface, giving a chemical concentration gradient from the outside to the center of the crystal, particularly where large sterically hindered molecules are concerned. Such observations have been made by Namba et al (Zeolites, 6, p 107 (1986)). Such preferred chemical distributions may be expected to have significant catalytic effects. Methods of directly synthesizing high silica faujasite would therefore be useful in optimizing both the zeolite product and the process for its production.
Although the disclosed ECR-4 composition is quite chemically and thermally stable in its own right because of its high silica content, that stability makes the inventive composition particularly useful as a starting material for the dealumination processes described above. Since the number of aluminum atoms in the framework of the inventive composition is lower than in zeolite Y, removal of these atoms causes less framework metastability during dealumination.
The use of tetramethyl ammonium cations (TMA) in the synthesis of zeolites A, Y and ZSM-4 (mazzite) is known, e.g., U.S. Pat. Nos. 3,306,922; 3,642,434; 4,241,036 and 3,923,639. In all these cases the TMA is trapped in the smaller cavities in the structures (sodalite or gmelenite cages), and must be burned out at high temperatures, often leading to lattice disruption and collapse. In most of these syntheses the SiO.sub.2 /Al.sub.2 O.sub.3 ratio of the zeolites is less than about 6.
In summary, the present invention is seen to provide a novel crystalline aluminosilicate, ECR-4, which has a faujasite structure and a silica to alumina ratio of at least six, and contains alkyl ammonium cations within its structure which can be readily removed at relatively low calcination temperatures. It may be viewed as a major improvement over its chemically modified counterparts, from which it may be differentiated by considering:
o Development of non-selective mesopores PA1 o Occluded detrital Al and Si species PA1 o Residual F.sup.- anions PA1 o Crystal Si/Al composition gradients
all of which are absent in ECR-4. These undesirable features may be readily observed by a variety of instrumental techniques, including sorption measurements, catalytic properties, .sup.19 F, .sup.29 Si and .sup.27 Al-MASNMR, and microprobe analysis.
It is also known that even minor changes in the size or charge distribution of these large organic cations can induce the formation of different zeolite structures. U.S. Pat. No. 4,046,859 teaches that replacement of one of the methyl groups of the TMA compound with a hydroxy ethyl group causes the formation of a ferrierite-like phase (ZSM-21). Many such examples are enumerated by Barrer (Zeolites, 1981). The objective of the present invention is to develop faujasite preparation methods yielding high silica materials, where the organic templates are not locked into the small cavities in the structure, but are instead present in the large "super cages" from which they can be readily removed without disruption and degradation of the host lattice.